A polyvinyl acetal resin is one of the thermoplastic resins which has been commercially used as a polyol resin for various coatings, adhesives, binders for powdered magnetic substances used for a magnetic recording media, a treatment agent for fibers, and an interlayer for multilayers glass used as windshields in cars, and which has the characteristics of an excellent resistance to light, transparency, dispersity in pigments, toughness, and adhesion.
However, its ductility, solubility, curability, compatibility, and thermoplasticity must be further improved. Furthermore, in the case when a polyvinyl acetal resin alone is used, it does not have sufficient thermoplasticity. Accordingly, it is generally used with a plasticizer.
Although polyvinyl acetal resin mixed with a plasticizer can provide a soft film, there are problems in that the plasticizer would move dispersively to other materials coming into contact with the film, resulting in the deterioration of desired properties, and further, its water resistance would become poor, depending upon the type of plasticizer, resulting in the limitation on its applications.
Still further, resins having a higher performance and a higher quality have been being recently searched for in response to requirements for energy saving, resource conservation, and variations or advances in use.
Regarding the coating field, various new types of resin have been proposed in response to the use of new coating types such as water-based coatings, highly solid coatings, powder coatings, electro-deposition coatings, photocurable coatings, elastic coatings, etc.
Conventional resins are not sufficient in quality for such a field.
Conventionally utilized resins have included alkyd resins, oil-free polyester resins, polyester polyols, polyester polyols, epoxy resins, and acrylpolyol resins as the resin for coatings curable at ordinary temperatures or in heating by mixing with a curing agent which can react with hydroxyl groups.
Resins have been used by mixing with a curing agent which can react to hydroxyl groups such as polyisocyanates, blocked polyisocyanates, melamine resins, phenol resins, epoxy resins, dialdehyde resins, and further, cellulosic resins such as cellulose nitrates, cellulose acetatebutylates, polyvinyl chloride resins, polyvinyl acetate resins etc., and pigments, solvents, additives, etc. to obtain coating compositions in various uses.
Of the resins having hydroxyl groups, alkyd resins have secondary hydroxyl groups which are poor in reactivity to curing agents, and it is exceedingly difficult to increase the concentration and molecular weight of hydroxyl groups in the resins at the same time during preparation.
Furthermore, alkyd resins have disadvantages of being poor in water resistance and weatherability. Still further, although oil-free alkyd resins and polyester polyols are used for many urethane coatings because of their primary hydroxyl groups which are high in reactivity, they have the disadvantage of poor water resistance because of the ester bonds they contain. In addition, a quick drying property and rigid coating layers cannot be readily obtained in the oil-free alkyd resins and polyester polyols.
Polyether polyols, which are prepared by the ring-opening polymerization of propylene oxide or ethylene oxide with polyfunctional alcohols are also widely used in the fields of urethane foams and flooring materials, etc., in which water resistance is required. However, polyether polyols are almost never used in the coating field because of poor weatherability and heat resistance.
Recently, particularly, acrylpolyol resins, which are prepared by the radical copolymerization of a hydroxyalkyl (meth)acrylate such as 2-hydroxyethyl methacrylate with acrylates or styrene, are becoming more widely used for urethane coatings dried at ordinary temperatures or cured by baking, and melamine coatings cured by baking.
More specifically, acrylpolyol resins are used in coatings such as acryl urethane coatings for car body repair, baking coatings for electrical home appliances, metallic coatings for car bodies, coatings for the outside walls in buildings which require weatherability, resistance to chemicals, resistance to water, resistance to contamination, and high rigidity.
However, excellent coating layers cannot be obtained from the acrylpolyol resins, and hydroxyl groups can only partially react to curing agents because the hydroxyl groups connect in the vicinity of the main structure of acryl polyol resins.
Although oligomers having a high molecular weight derived from epibis type epoxy resins also have hydroxyl groups, they are secondary hydroxyl groups which are poor in reactivity. Furthermore, oligomers themselves are too rigid to use for coatings without a modification thereof.
Still further, for example, in the case where polyvinyl acetal resins are employed as binders for powdered magnetic substances used for a magnetic recording medium, although they are excellent in the dispersion of powdered magnetic substances, the resulting magnetic layers are insufficient in flexibility, resulting in the causing of curling during storage and being abraded by tape guides and magnetic heads during recording or playback.
For the purpose of solving the above-mentioned disadvantages, and as a result of an intensive investigation, the inventors of this invention have now found that it is possible to solve the above problems by introducing lactone units into a polyvinyl acetal resin.